Cold-work die steel and dies for cold pressing

ABSTRACT

The present invention provides a cold-work die steel useful as a material of dies for cold pressing, which has basic properties such as hardness, toughness and dimensional change by heat treatment, and besides, which causes no problem in terms of machined surface roughness and cutting tool life, and also its dies for cold pressing. The invention relates to a cold-work die steel comprising: 0.5 to 0.7 mass % of C; 5.0 to 7.0 mass % of Cr; 0.5 to 2.0 mass % of Si; 0.1 to 2.0 mass % of Mn; 0.001 to 0.010 mass % of Al; 0.25 to 1.00 mass % of Cu; 0.25 to 1.00 mass % of Ni; 0.5 to 3.0 mass % of Mo+0.5×W; 0.5 mass % or less of V; 0.05 mass % or less of P; 0.1 mass % or less of S; 0.005 mass % or less of O, wherein the following requirements are satisfied: [C]×[Cr]≦4; FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0; and AP=[Mn]+3×([Cu]+[Ni])≦2.5, and also relates to a die for cold pressing which is manufactured by using the cold-work die steel.

TECHNICAL FIELD

The present invention relates to a cold-work die steel useful as amaterial for dies for cold pressing, which are used in carrying outpress forming (stamping, bending, drawing, trimming, and the like) ofsteel plates for cars, steel sheets for home electric appliances and soon under cold working or the like, and further relates to its dies forcold pressing.

BACKGROUND ART

With increases in strength of steel plates and sheets, dies for coldpressing which are employed for press forming of steel plates for cars,steel sheets for home electric appliances and the like are required toundergo further improvement in their life. As to the steel plates forcar use in particular, for enhancement of fuel economy of cars withconsideration given to environmental issues, high-tensile steel plateshaving tensile strengths of 590 MPa or more have come to be adopted ingrowing numbers, and it is conceivable that its demand will increasemore and more from now on.

In carrying out the press forming of high-tensile steel plates, there isan increase in the frequency of occurrence of a problem that the surfacecoating film of a surface-treated die for cold pressing suffers damageat an early stage and thereby a seizing-up phenomenon referred to ascompact attach or galling is caused during the press forming to resultin extreme loss of life to the die for cold pressing.

A die for cold pressing is manufactured by giving hard coating treatmentto the surface of a cold-work die steel as a base material. Thecold-work die steel as a base material is generally manufactured byundergoing processes of heat treatment or annealing, cut working andquenching-tempering treatment in order of mention.

As the cold-work die steel, not only high-C, high-Cr alloy tool steel,such as JIS SKD11, but also high-speed tool steel having furtherimproved abrasion resistance, such as JIS SKH51, has generally been usedin the past. Improvement in hardness of these tool steels are made bygiving them precipitation hardening of Cr carbide or Mo, W or V carbide.In addition, low-alloy high-speed tool steels referred to as matrix highspeed steels which are improved in both toughness and abrasionresistance by reducing contents of alloy elements contained in JISSKH51, such as C, Mo, W and V, are used as cold-work die steels.Further, there are proposals of the arts disclosed in Patent Document 1and Patent Document 2 with the intention of further improving propertiesof those cold-work die steels.

With the intension of attaining excellent properties of inhibitingdimensional change and securing high hardness and galling resistancewithout impairment of the required properties such as machinability andabrasion resistance, Patent Document 1 discloses a cold-work die steelwhich allows carbide to have finely-dispersed distribution in itstexture by adding Ni and Al in proper amounts, and further by adjustingC and Cr contents in concert with Cu addition in an amount appropriateto the amounts of Ni and Al added.

On the other hand, with the intention of attaining properties, such ashardness after heat treatment and toughness, on the same levels as thoseof conventional matrix high speed steels even when quenching isperformed at temperatures lower than those adopted for the conventionalmatrix high speed steels, Patent Document 2 discloses the alloy toolsteel that has a microstructure in which 2 to 5 vol % of M₂₃C₆-typecarbide is formed under tempered conditions, and besides, that has amicrostructure including at least either MC-type carbide or M₆C-typecarbide precipitated in a dispersed state after quenching-tempering.

Patent Document 1: JP-A-2006-169624

Patent Document 2: JP-A-2004-169177

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

A die for cold pressing is manufactured by giving hard coating treatmentto the surface of a cold-work die steel as a base material. Examples ofthe hard coating treatment include TD treatment by which a coating filmcomprising VC is formed through thermal diffusion, CVD treatment bywhich a coating film mainly comprising TiC is formed, and PVD treatmentby which a coating film mainly comprising TiN is formed. These hardcoating treatments are adopted as appropriate according to thecircumstances of die users and press makers. Therefore, it is requiredto develop cold-work die steels adaptable to any of hard coatingtreatments. In addition, as a matter of course, dies for cold pressingare also required to ensure basic properties including hardness,toughness and dimensional change by heat treatment.

Dies for cold pressing have an additional problem of suffering fromplucking during cut working. When the plucking occurs, roughness of themachined surface becomes great, wrapping operation after heat treatmentbecomes difficult, and besides, reduction in die life is caused. Inaddition, the cutting tool life is shortened and the production cost isincreased. For solving these problems, it is required to inhibitprecipitation of Al inclusions (Al₂O₃, AlN) as a cause of the problemoccurrence. However, there is a fear that reduction in the content of Alas an element causing precipitation of Al inclusions affects ratheradversely the basic properties such as reduction of hardness, reductionof toughness and increase in dimensional change by heat treatment. Underthese circumstances, it has been awaited to develop dies for coldpressing, which can ensure those basic properties, and besides, whichhave no problems from the viewpoints of roughness of machined surface,cutting tool life and so on.

The invention has been made in order to solve these conventionalproblems, and subjects thereof are to provide a cold-work die steeluseful as a material of dies for cold pressing, which has not only basicproperties required, such as hardness, toughness and dimensional changeby heat treatment, but also adaptability to various types of hardcoating treatment, and besides, which causes no problems in terms ofmachined surface roughness and cutting tool life, and to provide itsdies for cold pressing.

Means for Solving the Problems

The gist of the invention is described below.

[1] A cold-work die steel comprising:

0.5 to 0.7 mass % of C;

5.0 to 7.0 mass % of Cr;

0.5 to 2.0 mass % of Si;

0.1 to 2.0 mass % of Mn;

0.001 to 0.010 mass % of Al;

0.25 to 1.00 mass % of Cu;

0.25 to 1.00 mass % of Ni;

0.003 to 0.025 mass % of N;

more than 0 to 0.05 mass % of P;

more than 0 to 0.1 mass % of S;

more than 0 to 0.005 mass % of O; and

at least one of Mo and W,

with a remainder comprising iron and an unavoidable impurity;

wherein the following requirements are satisfied:

0.5≦[Mo]+0.5×[W]≦3.0;

[C]×[Cr]≦4;

FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0; and

AP=[Mn]+3×([Cu]+[Ni])≦2.5;

wherein FP is a parameter associated with the ferrite-forming elements,AP is a parameter associated with the austenite-forming elements, andthe bracket means a content (mass %) of an element written therein.

[2] The cold-work die steel according to [1], further comprising morethan 0 to 0.5 mass % of V.[3] The cold-work die steel according to [1] or [2], further comprisingat least one element selected from the group consisting of Ti, Zr, Hf,Ta and Nb in a total content of more than 0 to 0.5 mass %.[4] The cold-work die steel according to any of [1] to [3], furthercomprising more than 0 to 10 mass % of Co.[5] A die for cold pressing, which is manufactured by working thecold-work die steel according to any of [1] to [4] and giving surfacetreatment thereto.

ADVANTAGES OF THE INVENTION

The use of the cold-work die steel of the invention as a material fordies for cold pressing allows provision of dies for cold pressing, whicheach have not only basic properties required, such as hardness,toughness and dimensional change by heat treatment, but alsoadaptability to various types of hard coating treatments, and besides,which cause no problems in terms of roughness of machined surface andcutting tool life. Moreover, the dies for cold pressing made by use ofthe cold-work die steel are particularly suitable for use in forminghigh tensile steel plates having tensile strength of 590 MPa or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a mechanism for damaging a TiN coating film by Crcarbide. Therein, (a) is a vertical cross-section view showing anoriginal die for cold pressing, (b) is a vertical cross-section viewshowing a state in which cracks are formed in the TiN coating film ofthe die for cold pressing, and (c) is a vertical cross-section viewshowing a state in which exfoliation of the TiN coating film occurs fromthe cracks as starting points.

FIG. 2 is an explanatory diagram representing a Charpy impact test pieceused for determining Charpy impact values in Examples.

FIG. 3 is an explanatory diagram demonstrating heat treatment conditionsadopted in giving heat treatment to test samples used for determining amaximum rate of dimensional changes by heat treatment in Examples.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1: Cold-work die steel    -   2: TiN coating film    -   3: Cr carbide    -   4: Crack

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described below in more detail on the basis of itsillustrative embodiments.

In the first place, the inventors have researched on causes of damage toa TiN coating film formed by PVD treatment and occurrence of galling indies for cold pressing which were made by using as a materialconventional JIS SKD11 and matrix high speed steels.

As a result of the research, it has been found that a cause ofoccurrence of galling in the TiN coating film lay in coarse Cr carbideformed in the cold-work die steels used as a base material, and the Crcarbide formed the starting point of occurrence of galling. Themechanism of damage of the TiN coating film by the Cr carbide is asshown in FIG. 1.

As shown in FIG. 1( a), to begin with, the surface of a cold-work diesteel 1 to be used as a base material is subjected to hard coatingtreatment, thereby preparing a die for cold pressing in which a TiNcoating film 2 is formed on its surface. When this cold-work die steel 1is formed using as a material JIS SKD11 or a matrix high speed steel,coarse Cr carbide 3 precipitates out at the surface of the cold-work diesteel 1 as the base material. In carrying out press forming by use ofthis die for cold pressing, as shown in FIG. 1( b), cracks 4 are formedin the TiN coating 2 by sliding the material in the direction of thearrow. The sites at which cracks 4 are formed lie in areas where Crcarbide 3 precipitates out in the base material beneath the TiN coatingfilm 2. When the material is further slid, as shown in FIG. 1( c), thecracks 4 act as starting points and cause exfoliation in the TiN coatingfilm 2, resulting in occurrence of galling.

As mentioned above, the cause of galling occurrence in the TiN coatingfilm lies in Cr carbide. The inventors have found that exfoliation ofthe TiN coating film can be prevented by inhibiting formation of the Crcarbide, and thereby a problem of extreme reduction in die life can beinhibited from arising.

In order to inhibit formation of coarse Cr carbide 3 precipitating outat the surface of a cold-work die steel to be used as a base material,thereby increasing the lifetime of a TiN coating film formed by PVDtreatment, it is appropriate to reduce both C content and Cr content inthe steel. However, an excessive reduction in C content makes itdifficult to form a VC coating film by TD treatment and a TiC coatingfilm by CVD treatment on the surface of the cold-work die steel.Therefore, the invention makes it possible to prevent coarse Cr carbide3 from precipitating out at the surface of a cold-work die steel, andmoreover to form a VC coating film and TiC coating film having necessaryand sufficient thicknesses on the surface of a cold-work die steel bynot only controlling the C content to be from 0.5 to 0.7 mass % and theCr content to be from 5.0 to 7.0 mass % but also specifying the productof these contents.

In addition, the invention specifies both a parameter associated withferrite-forming elements, such as Si, Cr, Mo, W, V and Al, and aparameter associated with austenite-forming elements, such as Mn, Cu andNi.

When the total content of ferrite-forming elements, such as Si, Cr, Mo,W, V and Al, is too high, not only a balance between hardness andtoughness of a cold-work die steel is lost, but also accuracy ofworked-machined surface becomes worse. Therefore, the invention performsmathematization of the parameter (FP) associated with theferrite-forming elements, and further adjusts the total content offerrite-forming elements to satisfy the mathematic expression, andthereby the invention ensures not only a good balance between hardnessand toughness in the cold-work die steel but also improvement inaccuracy of worked-machined surface.

On the other than, when the total content of austenite-forming elements,such as Mn, Cu and Ni, is too high, there occurs an increase in residualaustenite content to result in not only a wide variation in rate ofdimensional change by heat treatment but also a reduction in tool lifeunder cutting. Therefore, the invention performs mathematization of theparameter (AP) associated with the austenite-forming elements, andfurther adjusts the total content of the austenite-forming elements tosatisfy the mathematic expression, and thereby not only the amount ofresidual austenite in the steel is reduced to result in a narrowvariation in rate of dimensional change by heat treatment but also thetool life under cutting is increased.

Reasons for specifying the content ranges of chemical components in thecold-work die steel of the invention are described in detail on anelement basis. Additionally, all percentages in the presentspecification mean percent by mass.

C: 0.5 to 0.7%

C is an element that ensures hardness and abrasion resistance andcontributes to inhibition of HAZ softening. Additionally, when a carbidecoating film, such as a VC coating film by TD treatment or a TiC coatingfilm by CVD treatment, is formed on the surface of a base material forthe die, a low content of C therein causes a problem that the coatingfilm formed has a small thickness, and so on. Considering thesecircumstances, the lower limit of the content of C is set to 0.5% forthe purpose of effectively fulfilling the above functions. And the lowerlimit thereof is preferably 0.55%. However, an excessive content of Ccauses production of coarse Cr carbide and makes it easy for a TiNcoating film formed by PVD treatment to exfoliate. In addition, anexcessive content of C causes an increase in residual austenite content,as a result, the desired hardness cannot be attained unless temperingtreatment is performed at a high temperature, and besides, a greatdimensional change occurs through expansion or the like after thetempering treatment. Moreover, an excessive content of C affectsadversely the toughness. Therefore, the upper limit of the content of Cis set to 0.7%. And the upper limit thereof is preferably 0.65%.

Cr: 5.0 to 7.0%

Cr is an element useful for ensuring the proper hardness. Specifically,a too low content of Cr brings about insufficient hardenability duringquenching and leads to partial production of bentonite, as a result, thehardness is lowered, and the abrasion resistance cannot be secured.Moreover, Cr is an element useful also for ensuring corrosion resistanceof dies. Therefore, the lower limit of the content of Cr is set to 5.0%.And the lower limit thereof is preferably 5.5%. However, an excessivecontent thereof causes an increased production of coarse Cr carbide andmakes it easier for a TiN coating film formed by PVD treatment toexfoliate. In addition, an excessive content of Cr causes a reduction indurability of the hard coating film through shrinkage after heattreatment. Moreover, an excessive content of Cr affects adversely thetoughness. Therefore, the upper limit of the content of Cr is set to7.0%. And the upper limit thereof is preferably 6.5%.

Si: 0.5 to 2.0%

Si is useful as a deoxidizing element at the time of steelmaking, and isan element that contributes to a hardness improvement and ensuresmachinability. In addition, Si is useful for inhibiting the softening ofmartensite in a matrix by tempering and inhibiting HAZ softening. Forthe purpose of fulfilling such functions effectively, the lower limit ofthe content of Si is set to 0.5%. Additionally, the content thereof ispreferably 1.0% or more, more preferably 1.2% or more. However, anexcessive content thereof brings about a reduction in toughness. Inaddition, increases in segregation and dimensional change after heattreatment are caused. Therefore, the upper limit of the content of Si isset to 2.0%. And the content thereof is preferably 1.85% or less.

Mn: 0.1 to 2.0%

Mn is an element useful for securing hardenability during quenching.

However, an excessive content thereof brings about an increase inresidual austenite content, as a result, the desired hardness cannot beattained unless tempering treatment is performed at a high temperature,and besides, the toughness is lowered. Considering these circumstances,the content of Mn is so specified that the range thereof is between 0.1%and 2.0%. The lower limit of the content of Mn is preferably 0.15%, andthe upper limit is preferably 1.0%, more preferably 0.5%, further morepreferably 0.35%.

Al: 0.001 to 0.010%

Al is an element useful as a deoxidizer. When the content thereof isless than 0.001%, the effect cannot be fully achieved. Therefore, thelower limit of the content of Al is set to 0.001%. And the lower limitthereof is preferably 0.002%. On the other hand, Al inclusions, such asAl₂O₃ and coarse AlN, become a cause of the plucking during cutting, andaggravate accuracy of worked-machined surface. Accordingly, the upperlimit of the content of Al is set to 0.010%. And the upper limit thereofis preferably 0.008%.

Cu: 0.25 to 1.00%

Cu is an element necessary to aim at hardness improvement byprecipitation hardening of ε-Cu, and contributes also to inhibition ofHAZ softening. However, an excessive content thereof causes a reductionin toughness, and tends to cause forging cracks. Therefore, the upperlimit of the content of Cu is set to 1.00%. And the upper limit thereofis preferably 0.80%. Further, the lower limit of the content of Cu isset to 0.25%. And the lower limit thereof is preferably 0.30%.

Ni: 0.25 to 1.00%

Ni is an element necessary to aim at hardness improvement byprecipitation hardening of an Al—Ni intermetallic compound, such asNi₃Al, and contributes also to inhibition of HAZ softening. In addition,the use of Ni in combination with Cu allows control of hot embrittlementby Cu addition in an excessive amount, and thereby the forging crackscan also be prevented. However, an excessive content thereof causes anincrease in residual austenite content, as a result, the proper hardnesscannot be attained unless tempering treatment is performed at a hightemperature, and besides, expansion occurs after heat treatment. Inaddition, an excessive content of Ni causes a reduction in toughness.Considering these circumstances, the content of Ni is so specified thatthe range thereof is between 0.25% and 1.00%. The lower limit of thecontent of Ni is preferably 0.30%, and the upper limit thereof ispreferably 0.80%.

N: 0.003 to 0.025% N is an important element for attainment of excellenttoughness by formation of AlN precipitates in conjunction with Al andprevention of grain growth during quenching. For the purpose ofattaining excellent toughness, the lower limit of the content of N isset to 0.003%. And the lower limit thereof is preferably 0.004%. Inaddition, the upper limit of the content of N is 0.025%. And the upperlimit thereof is preferably 0.017%.

Mo+0.5W: 0.5 to 3.0%

Mo and W are elements that contribute to precipitation hardening becauseeach of Mo and W forms M₃C-type carbide or M₆C-type carbide, andbesides, forms Ni₃Mo intermetallic compound or the like. However,excessive contents of these result in overproduction of those carbidesand so on, which leads to not only a reduction in toughness but also anincrease in dimensional change after heat treatment. Therefore, the sumof content of Mo and content of W, to which the expression Mo+0.5W isapplied, is specified so as to fall in a range of 0.5 to 3.0%.Additionally, the content of Mo in itself is preferably in a range of0.5 to 3.0%. On the other hand, the content of W in itself is preferably2.0% or less (including 0%). In other words, Mo is an essential element,while W is an optional element. However, the lower limit of the contentof W in itself is preferably 0.02%. Moreover, it is more preferred thatthe lower limit of the content of Mo in itself be a lower limit of 0.7%and an upper limit thereof be 2.5%. On the other hand, it is morepreferred that the lower limit of the content of W in itself be 0.05%and the upper limit thereof be 1.5%.

P: more than 0 to 0.05%

P is an element that is unavoidably present in dissolved raw materials,and that impairs toughness. Therefore, the upper limit of the content ofP is set to 0.05%. And the upper limit thereof is preferably 0.02%.Although the lower the content of P is, the better it is, the inclusionof P is unavoidable, so the practical lower limit of the content thereofis about 0.005%.

S: more than 0 to 0.1%

S is an element useful for ensuring machinability. From the viewpoint ofensuring machinability, it is recommended that the content of S be0.002% or more, preferably 0.004% or more. However, an excessive contentthereof results in occurrence of welding cracks. Therefore, the upperlimit of the content of S is set to 0.1%. The upper limit of the contentof S is preferably 0.07%, more preferably 0.05%, further more preferably0.025%.

O: more than 0 to 0.005%

O is an element included in molten steel, and it is unavoidably presentin steel. When the content of O is high, it reacts with Si, Al and thelike to form oxide inclusions. Therefore, the O content is so specifiedthat its upper limit is 0.005%. The upper limit thereof is preferably0.003%, more preferably 0.002%. Additionally, the lower the content of Ois, the better it is, but inclusion of O is unavoidable, so thepractical lower limit thereof is about 0.0005%.

Furthermore, it is an essential condition for the invention to satisfyall of the mathematical expressions described above. Additionally, thebracket in each mathematical expression represents the content (mass %)of an element written therein.

[C]×[Cr]≦4

The above mathematical expression is a mathematical expression definedfor the purpose of inhibiting the production of coarse Cr carbide. Whenthe product of content of C and content of Cr is more than 4, thereoccurs not only degradation in durability of hard coating films but alsoincrease in dimensional change after heat treatment. From the viewpointsof inhibiting the formation of coarse Cr carbide and inhibiting thedimensional change after heat treatment, it is preferred that theproduct of content of C and content of Cr be minimized. However, furtherconsidering significant achievement of the effects from the addition ofC and Cr, the lower limit of the product is preferably basically 0.8.

FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0

The above mathematical expression is a mathematical expression definedby parameterizing the sum relating of the contents of ferrite-formingelements such as Si, Cr, Mo, W, V and Al. When this parameter (FP) ismore than 5.0, not only the balance between hardness and toughness ofthe cold-work die steel is lost, but also the accuracy ofworked-machined surface is aggravated. This parameter (FP) is preferably4.8 or less. And the FP value of 2.11 determined from the lower limitvalues concerning the elements essentially included in the cold-work diesteel according to the invention, such as Si and Cr, is a substantiallower limit value of this parameter (FP).

AP=[Mn]+3×([Cu]+[Ni])≦2.5

The above mathematical expression is a mathematical expression definedby parameterizing the sum of the contents of austenite-forming elements,such as Mn, Cu and Ni. When this parameter (AP) is more than 2.5, theresidual austenite is increased in content, and thereby not only therate of dimensional change by heat treatment varies widely but also thetool life under cutting is shortened. This parameter (AP) is preferably2.3 or less. And the AP value of 1.6 determined from the lower limitvalues concerning Mn, Cu and Ni is a substantial lower limit of thisparameter (AP).

Requirements concerning the basic components in the cold-work die steelof the invention are as mentioned above. The remainder comprises ironand unavoidable impurities. As the impurities, e.g. Sn, Pb and so on areexemplified. In addition, for the purpose of improving other properties,the following optional components may be further included.

V: 0 to 0.5%

V contributes to an improvement in hardness by forming carbide such asVC, and besides, it is an element effective in inhibiting HAZ softening.In addition, when a diffusion hardening layer is formed by givingnitriding treatment, such as gas nitriding, salt bath nitriding orplasma nitriding, to the surface of a base material, it is an effectiveelement for improvement in surface hardness and increase in hardeninglayer depth. For significant achievement of those effects, it isappropriate that the content of V be basically 0.05% or more. However,an excessive content of V lessens the amount of C dissolved in solid andcauses a reduction in hardness of the martensite texture as a matrix,and besides, it reduces the toughness. Therefore, the upper limit of thecontent of V is set to 0.5%. And, the upper limit thereof is preferably0.4%, more preferably 0.3%.

At least one element selected from the group consisting of Ti, Zr, Hf,Ta and Nb: 0.5% or less in total

All of these elements are nitride-forming elements, and they contributeto a finely dispersed state of their nitrides and AlN, accordingly theyare elements allowing prevention of grain growth and contribution toimproved toughness. For significant achievement of such effects, it isbasically appropriate that 0.01% or more of Ti, 0.02% or more of Zr,0.04% or more of Hf, 0.04% or more of Ta and 0.02% or more of Nb becontained. However, when the total content of these elements becomesexcessive, the amount of C dissolved in solid is lessened to result in ahardness reduction of martensite. Therefore, these elements are sospecified that they have a total content of 0.5% or less. The totalcontent of these elements is preferably 0.4% or less, more preferably0.3% or less. Additionally, these elements may be contained alone or incombination with two or more thereof.

Co: 10% or less

Co is an element effective in heightening an Ms point and reducingresidual austenite, and thereby enhancing the hardness. For significantachievement of such effects, it is basically appropriate that thecontent of Co be 1% or more. However, an excessive content thereofbrings about rises in cost and so on. Therefore, the upper limit of thecontent of Co is set to 10%. The upper limit of the Co content ispreferably 5.5%. Herein, the term “Ms point” is one of transformationtemperatures (a temperature at which a phase change occurs, or atemperature at which transformation begins or comes to an end when thetransformation lasts over a temperature range), and refers to thetemperature at which austenite begins undergoing transformation intomartensite during cooling.

Dies for cold pressing are manufactured by using the cold-work die steelthat satisfies the requirements described above. An example of a methodof manufacturing such dies for cold pressing is explained below. Forinstance, after production by melting, the cold-work die steel of theinvention is subjected to hot forging, and then softened by undergoingannealing (e.g. by being kept at about 700° C. for 7 hours, and thensubjected to cooling to about 400° C. in furnace at an average coolingrate of about 17° C./hr, and further to standing to cool). Thereafter,the resultant is crude-processed into intended forms by e.g. a cuttingwork, and then hardened so as to acquire an intended hardness byundergoing quenching at temperatures ranging from 950° C. to 1,150° C.and further by tempering at temperatures ranging from 400° C. to 530° C.Thus, dies for cold pressing are manufactured.

EXAMPLES

Now, the invention will be illustrated in more detail by reference tothe following examples, but the invention should not be construed asbeing limited to these examples.

In these examples, steel species having chemical compositions listed inTable 1 with 26 varieties in total (No. 1: JIS SKD11 conventionally usedas a cold-work die steel) were used. From each of the steel species, 150kg of ingot was produced by melting in a vacuum induction meltingfurnace, and then heated to a temperature of 900° C. to 1,150° C. andthereby forged into a plate having a size of 40 mmT×75 mmW×about 2,000mL. Thereafter, each plate obtained was slowly cooled at an averagecooling rate of about 60° C./hr. After cooling to a temperature of 100°C. or less, re-heating to a temperature of about 850° C. and subsequentslow cooling at an average cooling rate of about 50° C./hr (heattreatment or annealing) were carried out. Various tests were made oneach of the heat-treated or annealed materials thus obtained.

(1) Determination of Maximum Hardness

A test piece having a size of 20 mmT×20 mmW×15 mmL was cut from each ofthe heat-treated or annealed materials, and used as the test specimenfor hardness measurement. Each test specimen was subjected to heattreatment, and more specifically, it underwent such treatment thatquenching (heating at 1,030° C. for 120 minutes), air cooling, tempering(keeping for 180 minutes in a temperature range of 450° C. to 520° C.)and standing to cool were carried out in order of mention. Hardnessmeasurements thereof were made with a Vickers hardness tester(manufactured by ΛKΛSHI Co., Ltd., ΛVK standard, load of 5 kg) as thetempering temperature was shifted within the range of 450° C. to 520°C., and the maximum hardness thereof was determined. The test specimensshowing maximum hardness of 650 HV or more in these measurements wereregarded as acceptable. The test results are shown in Table 2.

(2) Measurement of Charpy Impact Value (Toughness Measurement)

Each of the heat-treated or annealed materials underwent heat treatment,specifically such treatment that quenching (heating at 1,030° C. for 120minutes), air cooling, tempering (keeping for 180 minutes in atemperature range of 450° C. to 520° C.) and air cooling or standing tocool in order of mention. A test piece having an R-notch section of10-mm R as shown in FIG. 2 was cut, and used as a test specimen fortoughness measurement (Charpy Impact test specimen). Charpy impacttesting was carried out on this specimen, and absorption energy at roomtemperature (Charpy impact value) was determined. From each steelspecies, three test species for Charpy impact testing were taken, andthe average value thereof was taken as Charpy impact value. In thistesting, when the Charpy impact value obtained by measurement was 20 Jor more, the test specimens having such Charpy impact values wereregarded as acceptable. These test results are shown in Table 2.

(3) Survey on Machined Surface Roughness

Each of the heat-treated or annealed materials was used as a testsample, subjected to finish working by means of a ball end mill, andexamined for machined surface roughness. Testing conditions adopted wereas follows.

Machine: MORI (BT40, 5.5 kw)

Tool: Mitsubishi SRFH30S32M φ30

Tip: Mitsubishi SRFT30 VP10MF φ30

Projection length: 118 mm

Cutting direction: Down cut

Cutting rate: 250 mm/min

Feed rate: 0.31 min/rev

Cut: Ad 0.3 mm, Rd 0.7 mm

Cutting oil: Nothing (air-blow)

Working distance: 257.1 m

The machined surface roughness Ra is defined as the average value of thevalues obtained by carrying out surveys at 5 points chosen from a lengthrange of 10 mm in each test sample. In this testing, the test sampleshaving machined surface roughness Ra of 0.40 mm or more were regarded asacceptable. Test results obtained are shown in Table 2.

(4) Determination of Cutting Tool Life Each of the heat-treated orannealed materials was used as a test sample, subjected to crude workingwith a high feed cutter, and examined on the cutting tool life. Testingconditions adopted were as follows.

Machine: OKK (BT50, 7.5 kw)

Tool: Mitsubishi AJX148R503SA42S φ50

Tip: JOMW140520ZDSR-FT VP15TF

Cutting rate: 10 m/min

Feed ratio: 1.0 mm/rev

Cut: Ad 1 mm, Rd 35 mm

Projection length: 80 mm

Cutting oil: Nothing (air-blow)

Life determination: Tool wear, chipping

The cutting tool life in the case of using each of the test samples andcarrying out the crude working was determined by examining how manytimes it was longer than the cutting tool life in the case of using thetest sample (No. 1) made from JIS SKD11 as a material and carrying outthe crude working, with the latter cutting tool life being taken as “1”.When the values determined were 4.0 or more, the test samples concernedwere regarded as acceptable. Test results obtained are shown in Table 2.

(5) Measurement on Maximum Rate of Dimensional Changes by Heat Treatment

Six blocks each having a size of 40 mmT×75 mmW×100 mL were cut from eachof the heat-treated or annealed materials, used as test samples formeasurement on maximum rate of dimensional changes by heat treatment,and subjected to heat treatment under the conditions as shown in FIG. 3.The maximum rate of dimensional changes by heat treatment was determinedfrom the rates of changes caused in dimensions of 6 test samples betweenbefore and after heat treatment. More specifically, each of the testsamples was measured on the rates of dimensional changes in orthogonalthree directions (x, y and z directions), and the maximum numericalvalue among the absolute values of 3×6 dimensional change rates measuredwas defined as the maximum rate of dimensional changes by heattreatment. In this testing, every case where the maximum rate ofdimensional changes by heat treatment was 0.08 or less was regarded asacceptable. Test results obtained are shown in Table 2.

TABLE 1 Chemical Component (mass %) No. Classification C Si Mn P S Al NiCu Cr Mo W  1 Comparative Example 1.49 0.35 0.42 0.018 0.005 0.050 0.080.05 12.10 1.04 0.35  2 Comparative Example 1.01 1.06 0.60 0.019 0.0070.330 0.44 0.40 8.38 0.91 0.39  3 Comparative Example 0.25 1.32 0.280.018 0.004 1.091 2.95 3.01 4.95 1.20 0.02  4 Comparative Example 0.401.35 0.25 0.019 0.004 1.030 2.98 3.00 4.45 1.21 0.02  5 ComparativeExample 0.60 1.00 0.40 0.020 0.004 0.009 0.67 0.04 5.87 0.93 0.02  6Comparative Example 0.58 0.95 0.42 0.018 0.004 0.0008 0.30 0.30 5.950.95 0.02  7 Example 0.58 0.98 0.42 0.019 0.005 0.002 0.29 0.30 5.970.95 0.02  8 Example 0.60 0.97 0.42 0.018 0.004 0.005 0.29 0.30 5.960.95 0.02  9 Example 0.59 0.98 0.43 0.019 0.004 0.009 0.30 0.30 5.970.96 0.02 10 Comparative Example 0.60 0.96 0.41 0.018 0.005 0.017 0.300.30 5.96 0.95 0.02 11 Example 0.58 1.70 0.42 0.018 0.004 0.003 0.300.29 5.95 0.95 0.02 12 Comparative Example 0.59 0.99 1.10 0.019 0.0040.003 0.30 0.30 5.96 0.95 0.02 13 Comparative Example 0.60 0.98 0.420.018 0.004 0.003 0.75 0.73 5.96 0.95 0.02 14 Example 0.58 0.98 0.430.019 0.004 0.003 0.30 0.30 5.97 1.70 0.02 15 Example 0.59 0.97 0.410.018 0.080 0.003 0.30 0.30 5.96 0.95 0.02 16 Example 0.60 0.99 0.420.019 0.004 0.003 0.29 0.28 5.95 0.96 0.02 17 Example 0.58 0.97 0.410.018 0.004 0.003 0.29 0.29 5.95 0.95 0.02 18 Example 0.60 0.98 0.400.019 0.004 0.003 0.30 0.30 5.96 0.96 0.02 19 Example 0.58 0.97 0.420.019 0.005 0.003 0.29 0.29 5.95 0.95 0.02 20 Example 0.59 0.98 0.410.018 0.004 0.003 0.29 0.30 5.97 0.95 0.02 21 Comparative Example 0.582.11 0.42 0.019 0.004 0.003 0.30 0.29 5.96 0.96 0.02 22 ComparativeExample 0.60 0.98 2.02 0.018 0.005 0.003 0.30 0.30 5.97 0.96 0.02 23Comparative Example 0.58 0.97 0.41 0.019 0.004 0.003 1.48 1.49 5.96 0.950.02 24 Comparative Example 0.60 0.99 0.42 0.019 0.004 0.003 0.29 0.305.95 0.19 0.19 25 Comparative Example 0.58 0.98 0.43 0.019 0.005 0.0030.30 0.30 5.95 2.97 0.05 26 Comparative Example 0.59 0.98 0.41 0.0180.004 0.003 0.29 0.28 5.96 0.96 0.02 27 Comparative Example 0.58 0.960.42 0.019 0.005 0.003 0.29 0.28 5.97 0.96 0.02 Chemical Component (mass%) [Mo] + [Cr] × No. Classification V Ti Nb Zr Hf Ta Co N O [W]/2 [C] FPAP  1 Comparative Example 0.25 0 0 0 0 0 0 0.0130 0.0015 1.22 18.03 5.920.81  2 Comparative Example 0.09 0 0.1 0 0 0 0 0.0068 0.0007 1.11 8.467.58 3.12  3 Comparative Example 0.20 0 0 0 0 0 0 0.0148 0.0014 1.211.24 14.98 18.16  4 Comparative Example 0.20 0 0 0 0 0 0 0.0165 0.00141.22 1.78 14.30 18.19  5 Comparative Example 0.32 0 0 0 0 0 0 0.01700.0015 0.94 3.52 3.98 2.53  6 Comparative Example 0.28 0 0 0 0 0 00.0162 0.0013 0.96 3.45 3.87 2.22  7 Example 0.28 0 0 0 0 0 0 0.01610.0014 0.96 3.46 3.89 2.19  8 Example 0.29 0 0 0 0 0 0 0.0165 0.00130.96 3.58 3.94 2.19  9 Example 0.28 0 0 0 0 0 0 0.0165 0.0014 0.97 3.523.98 2.23 10 Comparative Example 0.29 0 0 0 0 0 0 0.0162 0.0014 0.963.58 4.05 2.21 11 Example 0.28 0 0 0 0 0 0 0.0162 0.0013 0.96 3.45 4.042.19 12 Comparative Example 0.29 0 0 0 0 0 0 0.0161 0.0014 0.96 3.523.92 2.90 13 Comparative Example 0.28 0 0 0 0 0 0 0.0165 0.0014 0.963.58 3.90 4.86 14 Example 0 0 0 0 0 0 0 0.0162 0.0014 1.71 3.46 4.842.23 15 Example 0.28 0 0 0 0 0 0 0.0165 0.0014 0.96 3.52 3.90 2.21 16Example 0.28 0.04 0 0 0 0 0 0.0162 0.0013 0.97 3.57 3.92 2.13 17 Example0.29 0 0.1 0 0 0 0 0.0162 0.0014 0.96 3.45 3.91 2.15 18 Example 0.28 0 00.1 0 0 0 0.0162 0.0015 0.97 3.58 3.92 2.20 19 Example 0.29 0 0 0 0.10.1 0 0.0163 0.0014 0.96 3.45 3.91 2.16 20 Example 0.28 0 0 0 0 0 5.20.0161 0.0013 0.96 3.52 3.90 2.18 21 Comparative Example 0.28 0 0 0 0 00 0.0162 0.0014 0.97 3.46 4.14 2.19 22 Comparative Example 0.28 0 0 0 00 0 0.0165 0.0013 0.97 3.58 3.92 3.82 23 Comparative Example 0.29 0 0 00 0 0 0.0164 0.0014 0.96 3.46 3.92 9.32 24 Comparative Example 0.28 0 00 0 0 0 0.0165 0.0014 0.29 3.57 2.55 2.19 25 Comparative Example 0.28 00 0 0 0 0 0.0162 0.0013 3.00 3.45 7.97 2.23 26 Comparative Example 0.600 0 0 0 0 0 0.0164 0.0015 0.97 3.52 4.56 2.12 27 Comparative Example0.29 0 0 0 0 0 0 0.0254 0.0013 0.97 3.46 3.94 2.13 In the above table,compar. stands for Comparative Example, and ex. stands for Example.

TABLE 2 Maximum Charpy Impact Machined Surface Cutting Tool Life MaximumRate of Dimensional Hardness Value Roughness Ra ratio Changes by HeatTreatment No. Classification HV J mm (reference: No. 1) % 1 ComparativeExample 690 10 1.15 1 0.15 2 Comparative Example 720 13 0.85 1.5 0.11 3Comparative Example 685 22 0.52 3.5 0.08 4 Comparative Example 710 170.66 3.8 0.10 5 Comparative Example 700 15 0.20 4.5 0.09 6 ComparativeExample 720 19 0.19 4.4 0.07 7 Example 723 27 0.21 4.7 0.06 8 Example722 26 0.23 4.6 0.06 9 Example 724 27 0.29 4.5 0.05 10 ComparativeExample 726 30 0.41 4.6 0.07 11 Example 732 21 0.27 4.5 0.05 12Comparative Example 717 22 0.28 3.9 0.10 13 Comparative Example 720 250.29 3.6 0.12 14 Example 728 21 0.38 4.3 0.07 15 Example 720 20 0.18 5.00.08 16 Example 708 38 0.33 4.5 0.06 17 Example 710 38 0.34 4.3 0.07 18Example 705 36 0.37 4.3 0.06 19 Example 709 30 0.38 4.2 0.06 20 Example719 34 0.29 4.7 0.07 21 Comparative Example 725 15 0.25 4.8 0.08 22Comparative Example 727 14 0.28 3.5 0.09 23 Comparative Example 726 150.25 3.9 0.10 24 Comparative Example 645 18 0.23 4.7 0.05 25 ComparativeExample 742 15 0.39 4.2 0.06 26 Comparative Example 716 14 0.45 4.1 0.0627 Comparative Example 715 17 0.30 4.5 0.06 Acceptance Criteria ≧650 ≧20≦0.4 ≧4 ≦0.08

As listed in Table 1 and Table 2, each of Nos. 7 to 9, 11 and 14 to 20as Examples of the invention, which satisfies all of the requirements ofthe inventions for the contents of individual chemical components, theproduct of content of C and content of Cr, the parameter associated withferrite-forming elements and the parameter associated withaustenite-forming elements, had all of its maximum hardness, Charpyimpact value, machined surface roughness, cutting tool life and maximumrate of dimensional changes by heat treatment within the ranges of theirrespective acceptance criteria. By contrast, each of Nos. 1 to 6, 10, 12to 13 and 21 to 26 as Comparative Examples for the invention, which doesnot satisfy at least one of the requirements of the invention, missedmeeting at least one of the acceptance criteria, and had some problem.

Although it had some problem by missing out one or two or more of therequirements of the invention, each of Nos. 1 to 6, 10, 12 to 13 and 21to 26 as Comparative Examples was chosen as Comparative Example todistinguish one of the requirements from another. In the following, eachComparative Example is explained in relation to some of the requirementsspecified in the invention.

No. 1 and No. 2 are Comparative Examples in which both of the content ofC and the content of Cr are too high, and conversely, No. 3 and No. 4are Comparative Examples in which both of the content of C and thecontent of Cr are too low. All the Comparative Examples in which thosecontents and too high and too low were outside all or some of theacceptance criteria for the Charpy impact value (toughness), themachined surface roughness, the cutting tool life and the maximum rateof dimensional changes by heat treatment.

No. 21 is Comparative Example in which the content of Si is too high,and conversely, No. 1 is Comparative Example in which the content of Siis too low. No. 21 in particular, in which the content of Si is toohigh, was greatly reduced in toughness, and the Charpy impact valuethereof was outside the acceptance criterion. In addition, dimensionalchanges after heat treatment, though within the range of the acceptancecriterion, were relatively large.

No. 22 is Comparative Example in which the content of Mn is too high. Inthis Comparative Example, the toughness was greatly reduced, and theCharpy impact value was outside the acceptance criterion. In addition,the cutting tool life and the maximum rate of dimensional changes byheat treatment were also outside their individual acceptance criteria.

No. 10 is Comparative Example in which the content of Al is too high,and conversely, No. 6 is Comparative Example in which the content of Alis too low. In No. 10 as the Comparative Example in which the content ofAl is too high, plucking occurred when finish working was carried outwith a ball end mill, and thereby the accuracy of worked-machinedsurface was aggravated. On the other hand, No. 6 as the ComparativeExample in which the content of Al is too low, the Charpy impact valuewas outside the acceptance criterion.

No. 23 is Comparative Example in which the content of Cu is too high,and conversely, No. 5 is Comparative Example in which the content of Cuis too low. In No. 23 in which the content of Cu is too high, toughnesswas reduced, and the Charpy impact value thereof was outside theacceptance criterion. In addition, the cutting tool life and the maximumrate of dimensional changes by heat treatment were also outside theirindividual acceptance criteria. On the other hand, in No. 5 as theComparative Example in which the content of Cu is too low, the Charpyimpact value and the maximum rate of dimensional changes by heattreatment were also outside the acceptance criteria.

No. 23 is Comparative Example in which the content of Ni is too high,and conversely, No. 1 is Comparative Example in which the content of Niis too low. In No. 23 in which the content of Ni is too high, its Charpyimpact value and maximum rate of dimensional change by heat treatmentwere outside their individual acceptance criteria. In addition, thecutting tool life was also outside their individual acceptance criteria.

No. 24 is Comparative Example in which the numerical value determinedfrom Mo+0.5W is too small, and No. 25 is the case in which, although itlies in the range specified in the invention, the numerical value fallson the upper limit of 3.0%. In No. 24, the maximum hardness and theCharpy impact value were outside their individual acceptance criteria.On the other hand, in No. 25, the Charpy impact value was reduced thoughit was also influenced by missing out other requirements.

No. 26 is Comparative Example in which the content of V is too high. Inthis Comparative Example of No. 26, since the content of V was too high,the toughness was reduced, and the Charpy impact value was outside theacceptance criterion. In addition, the machined surface roughness wasoutside the acceptance criterion.

No. 1 and No. 2 are Comparative Examples in which the product of thecontent of C and the content of Cr is too great. Under this influence,in No. 1 and No. 2, cutting tool life was seriously reduced and thedimensional changes after heat treatment became great.

No. 27 is Comparative Example in which the content of Ni is too high. Asa result, the toughness was lowered, and the Charpy impact value wasoutside the acceptance criterion.

Nos. 1 to 4 and No. 25 are Comparative Examples in which the parameterassociated with the ferrite-forming elements is too great. Under thisinfluence, in Comparative Examples, toughness balance was lost and theaccuracy of worked-machined surface was aggravated. In No. 25 inparticular, in which only this requirement was outside, toughness wasseriously reduced, and the Charpy impact value thereof was outside theacceptance criterion.

Nos. 2 to 5, 12, 13, 22 and 23 are Comparative Examples in which theparameter associated with the austenite-forming elements is too great.Under this influence, in Comparative Examples, the residual austenitewas increased, and thereby not only the rate of dimensional change byheat treatment was increased but also the tool life under cutting wasshortened. In No. 12 and No. 13 in particular, in which only thisrequirement was outside, the cutting tool life and the maximum rate ofdimensional changes by heat treatment were outside the acceptancecriteria.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. This application is basedon Japanese Patent Application No. 2008-003524 filed on Jan. 10, 2008,and their contents are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The use of the cold-work die steel of the invention as a material fordies for cold pressing allows provision of dies for cold pressing, whicheach have not only basic properties required, such as hardness,toughness and dimensional change by heat treatment, but alsoadaptability to various types of hard coating treatments, and besides,which cause no problems in terms of roughness of machined surface andcutting tool life. Moreover, the dies for cold pressing made by use ofthe cold-work die steel are particularly suitable for use in forminghigh tensile steel plates having tensile strength of 590 MPa or more.

1. A cold-work die steel comprising: 0.5 to 0.7 mass % of C; 5.0 to 7.0mass % of Cr; 0.5 to 2.0 mass % of Si; 0.1 to 2.0 mass % of Mn; 0.001 to0.010 mass % of Al; 0.25 to 1.00 mass % of Cu; 0.25 to 1.00 mass % ofNi; 0.003 to 0.025 mass % of N; more than 0 to 0.05 mass % of P; morethan 0 to 0.1 mass % of S; more than 0 to 0.005 mass % of O; and atleast one of Mo and W, wherein the following requirements are satisfied:0.5≦[Mo]+0.5×[W]≦3.0;[C]×[Cr]≦4;FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦55.0; andAP=[Mn]+3×([Cu]+[Ni])≦2.5; wherein FP is a parameter associated with theferrite-forming elements, AP is a parameter associated with theaustenite-forming elements, and the bracket represents a content (mass%) of an element written therein.
 2. The cold-work die steel accordingto claim 1, further comprising more than 0 to 0.5 mass % of V.
 3. Thecold-work die steel according to claim 1, further comprising at leastone element selected from the group consisting of Ti, Zr, Hf, Ta and Nbin a total content of more than 0 to 0.5 mass %.
 4. The cold-work diesteel according to claim 1, further comprising more than 0 to 10 mass %of Co.
 5. A die for cold pressing, which is manufactured by working thecold-work die steel according to claim 1 and giving surface treatmentthereto.
 6. The cold-work die steel according to claim 2, furthercomprising at least one element selected from the group consisting ofTi, Zr, Hf, Ta and Nb in a total content of more than 0 to 0.5 mass %.7. The cold-work die steel according to claim 2, further comprising morethan 0 to 10 mass % of Co.
 8. The cold-work die steel according to claim3, further comprising more than 0 to 10 mass % of Co.
 9. The cold-workdie steel according to claim 6, further comprising more than 0 to 10mass % of Co.
 10. A process of making a die comprising working thecold-work die steel according to claim 1, and giving surface treatmentthereto.